The invention relates to an MRI apparatus for forming MR images of a patient to be examined in an imaging volume of the MRI apparatus, which apparatus is provided with:
means for generating a uniform magnetic field in the imaging volume of the MRI apparatus;
a patient support having a longitudinal axis extending in the direction of the uniform magnetic field; and
at least two RF receiving coils for receiving RF signals originating from the object, a first one of said RF receiving coils having a sensitivity vector which is directed transversely of the physical plane of said first coil.
An apparatus of the kind set forth is known from a publication in “Proceedings SMRM 1991”, p. 1240, entitled “Fast Imaging Method Using Multiple Receiver Coils with Subencoding Data Set”. The cited publication describes an imaging method which utilizes magnetic resonance and in which two RF receiving coils are used as the receiving antennas. The known method utilizes the fact that, as a result of the sub-sampling of the acquired magnetic resonance signals, less time is required to sample the k space across a region which is sufficiently large for the desired dimension (field of view) of the magnetic resonance image. More specifically, the respective lines in the k space along which sampling is performed are selected so as to be situated at distances from one another in the k space which are larger than necessary for the desired spatial resolution. One could say: “lines are skipped” in the k space. As a result of such “skipping of lines in the k space”, less time is required for the acquisition of the magnetic resonance signals. Receiving coil images are reconstructed on the basis of the sub-sampled magnetic resonance signals from the individual RF receiving coils. Such sub-sampling reduces the actual field of view so that backfolding or aliasing artifacts occur in said receiving coil images. The magnetic resonance image is derived from the receiving coil images on the basis of the sensitivity profiles of the RF receiving coils while eliminating the aliasing artifacts to a substantial degree or even completely from the magnetic resonance image. The magnetic resonance image is thus enlarged to the desired field of view.
An important requirement to be satisfied in eliminating the aliasing artifacts is that only a small degree of interdependence should exist between the sensitivity profiles of the RF receiving coils. The latter is not at all the case for identical receiving coils which fully overlap in space, and is so to a high degree when the planes of said coils extend perpendicularly to one another. The designer and/or user of the system, therefore, will always attempt to arrange the RF receiving coils in such a manner that their position and/or orientation approaches the desired mutual independence of the sensitivity profiles as much as possible.
Another requirement to be satisfied by the RF receiving coils consists in that the sensitivity vector of each RF receiving coil should extend as perpendicularly as possible to the uniform magnetic field in the imaging volume of the MRI apparatus. The above two requirements can be summarized as follows: it is desirable to have mutually independent sensitivity profiles of the RF receiving coils, meaning that the sensitivity vectors do not have a large component in common, and it is also desirable that the direction of said vectors also has a large component in the direction of the uniform magnetic field. Moreover, for a suitable signal yield the RF receiving coils should fit the part of the body of the patient to be imaged as well as possible.
The RF receiving coils in the MRI apparatus described in the cited publication in “Proceedings SMRM 1991” are situated in substantially the same plane, so that the above requirement concerning the small degree of interdependence is not satisfied.